The invention relates to a semiconductor element including at least one bi-polar power transistor, having parallel-connected transistor regions and partial base regions arranged in such a manner, that between active base regions at the emitter-base p n-junction and contacted base regions, resistances in the form of so-called base barrier resistances are provided.
During the operation of bi-polar power transistors it is of great importance, with respect to the operational reliability and life of such a component, to achieve uniform current distribution over the transistor as a whole.
Due to the negative temperature coefficient of the base-emitter voltage, a lateral thermal instability exists in bi-polar power transistors.
The following relationship approximately applies for the dependence of the collector current density J.sub.C on the temperature T and the base-emitter-voltage U.sub.BE : EQU J.sub.C =J.sub.Co X exp ((q.U.sub.BE -E.sub.g)/kT)
where:
J.sub.CO =Proportionality factor; PA1 q=Unit Charge (elementary charge); PA1 E.sub.g =band distance at silicone about 1.1 eV(energy level); PA1 k=Boltzmann constant.
and
If the temperature increases locally in a transistor relative to the surroundings, this results for a constant base-emitter voltage in an increase of the local current density according to the equation given above. This also leads to a local increase of the heat liberation as a power loss at the collector-base barrier layer and in the high-ohmic part of the collector. The resulting local temperature rise initiates a feedback coupling mechanism, resulting in a current restriction in the transistor. The high, local heat liberation associated therewith frequently causes irreversible damage to the transistor, leading to the failure of the component.
Such excessive temperatures must be prevented by a suitable current stabilization.
It is known, from the IEEE Journal of Solid State Circuits, Vol. SC-13, Nos. 3, pages 307 to 319, June 1978, to effect a current limitation in integrated power transistors by providing that the emitter zone is divided into several regions, whereby the part functioning mainly as an emitter is not contacted in the vicinity of the base connection, but is connected with contact-connected regions of the emitter zone through several part regions of the emitter zone which act as barrier resistances. This is done in such a configuration that each part-region of the effective emitter zone is associated with a barrier resistance and with a connection. An uniform current distribution over the individual partial regions of the effective emitter zone is achieved by the voltage drop over these barrier resistances, whereby the positive temperature coefficient of the emitter barrier resistance assists this regulation. Furthermore, because of the transistor construction described in the hereinafore-mentioned literature, a base barrier resistance is also provided besides the emitter barrier resistance. However, the base barrier resistance plays a minor role compared to the emitter barrier resistance. Furthermore, however, the insertion of emitter barrier resistances has the consequence that besides an additional power loss at the emitter barrier resistances, there is an increase in the collector-emitter-residual and saturation voltage.
It is accordingly an object of the invention to provide a semiconductor element and method for producing the same, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, and to provide a power transistor, which on one hand has a sufficiently uniform current distribution, and on the other hand exhibits a very low saturation and residual voltage, and has a power loss which is as low as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor element, comprising at least one bi-polar power transistor having parallel-connected transistor regions, active and contact connected partial base zones, arranged in a particular as described in detail below manner, an emitter zone-base zone pn-junction, and base barrier resistances disposed between the active non contact-connected base zones at the emitter-base pn-junction and the contact-connected base zones, the greater part of the base current being conducted through the base barrier resistances and the voltage drop over the emitter region being small compared to the voltage between the active base region and the contact-connected base zone. This feature can be made especially advantageous in cases where the voltage between the active and non-contact-connected base zone and the contact-connected ("contacted") base zone is about twice as great, or greater, than the voltage drop over the emitter region.
Instead of emitter resistances between the effective emitter region and the contacted emitter region, base barrier resistances are provided between the contacted base zones and active base regions, so that the same or an analogous effect is achieved as with the emitter barrier resistances. In other words, a uniform distribution of the base current and accordingly also of the emitter current is achieved. A positive temperature coefficient of the base barrier resistances assists this measure.
By providing base barrier resistances instead of emitter barrier resistances, an additional power loss at the emitter barrier resistances as well as an increase of the collector-emitter residual voltage and saturation voltage can advantageously be avoided simultaneously.
In accordance with another feature of the invention, the base barrier resistances are zones of the same doping type as the base zone connected between the contacted and active base regions. These barrier resistances can be effected by diffusions and implantations which are already contained in the manufacturing procedure used for producing the other structural components in the integrated circuit, so that no additional process steps are required.
In accordance with a further featuer of the invention, the base barrier resistances are in the form of narrowed portions of the base zone between the active and contacted base regions. By this measure the barrier resistances in the base can be provided simply and without additional work expenditure.
In accordance with an added feature of the invention, the narrowed portions of the base zone are generated by a suitable doping mask configuration. Thus, barrier resistances can be created by a suitable geometry of the doping-masks alone.
In accordance with an additional feature of the invention, the narrowed portions of the base zone are generated by providing additional doped zones in the base zone.
In accordance with again another feature of the invention, the additional doped zones are emitter doped.
It is also advantageous if, in the manufacture of a power transistor, there is provided a method for producing a semiconductor element power transistor which comprises introducing n.sup.+ -doped regions as a buried layer into a substrate, epitaxially depositing a layer above the buried layer and substrate, implanting isolation diffusions and n.sup.+ -doped collector deep-diffusions in an optional sequence in the epitaxially deposited layer, subsequently introducing p-dopings as a base zone and base barrier resistances into the epitaxial layer subsequently introducing n.sup.+ -dopings into the epitaxial layer as an emitter zone and to effect a good contact of the collector deep-diffusions, providing contact windows and providing conductor paths for making contact.
In accordance with a concomitant mode of the invention, which comprises providing the substrate in the form of p-doped silicon, and providing the layer above the buried layer and substrate in the form of n-doped silicon. This production method is especially suited for making integrated power transistors. The doping with respect to p and n can also be reversed.
Other features which are considered as characteristic for the invention are set forth below, in the drawings, and in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor element and method for producing the same, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.